I guess all kromo's asking for is a primary source. That's not the same as denying anything. It's just the "citation needed" of AB.
I think in either this article or a related one reference was made to "1,000 email complaints" about this sort of fraud. It's quite likely that many of these were complaints about the same incident. The basic point, again, is that I don't think there's any reason to believe that this was more than just a couple of isolated incidents. One student boasts of voting six times (apparently), many people pick up on this, and either exaggerate the scale or allow the impression to be drawn that this is only the tip of the iceberg.
Again, for the record, if indeed one student voted six times then I hope they are caught and punished appropriately. But also, again, any such case(s) of fraud was (were) certainly not the reason the Tories failed to win a majority.